The production of polarizing materials, especially plastic materials in sheet form, is well known in the art.
The production process generally comprehends two basic steps: first, a suspending medium containing long chain molecules, such a polyvinyl alcohol, is stretched to align those chain molecules; and, second, dichroic (polarizing) die molecules are added to (or included in) the medium which attach themselves in a manner so as to orient along the aligned chain molecules.
In an alternative two step process: first, acicular shaped light-polarizing particles are dispersed in a suspending medium; and, second, the medium is caused to flow under stress, such as by extruding, rolling, or stretching, in order to align the particles in the direction of medium flow.
Although the vast bulk of the polarizing materials marketed commercially has utilized an organic plastic as the suspending medium with organic and/or inorganic particles and/or molecules being dispersed therewithin, because of the inherent low surface hardness, relatively high moisture susceptibility, low temperature capability, and poor scratch resistance of organic plastics, investigations have been undertaken sporadically to manufacture polarizing glasses. Such research has been most active in the field of ophthalmic applications where high surface hardness and good scratch resistance are important.
Polarizing glasses have been prepared wherein very fine metallic particles and inorganic crystals have comprised the polarizing materials. Three methods for making polarizing glasses have been disclosed in the recent patent literature. U.S. Pat. Nos. 4,125,404 and 4,125,405 describe the preparation of photochromic and non-photochromic polarizing glasses effected through the irradiation of silver-containing glasses with intense polarized light. Photochromic polarizing glasses are generally clear in the undarkened state and polarizing in the darkened state. U.S. Pat. Nos. 3,653,863 and 4,282,022 disclose the manufacture of photochromic polarizing glasses via elongating, stretching or extruding the glass. U.S. Pat. No. 4,304,584 describes the preparation of polarizing glasses by elongating glasses containing silver halide particles, then heat treating said glass in a reducing atmosphere.
The first method involves subjecting a glass, wherein a silver halide selected from the group of AgCl, AgBr, and AgI constitutes the polarizing agent, to a high intensity source of linearly-polarized visible light, such as a laser, while the glass is in the darkened or colored state. The practical aspects of providing such an exposure to the glass have rendered the process intrinsically expensive and slow. Furthermore, the polarizing effect produced in this manner deteriorates on exposure to sunlight and the reported dichroic ratios are relatively low, &lt;4.
The second method is specifically directed to photochromic glasses wherein a silver halide comprises the photochromic agent. The technique contemplates stretching or extruding the photochromic glasses, while at temperatures between the annealing and softening points of the glass, to simultaneously elongate the silver halide particles to an ellipsoidal configuration (conventionally demonstrating a length-to-width ratio, termed the "aspect ratio", ranging between about 2:1-5:1 for stretching and 2:1-30:1 for extrusion) and align the elongated particles. The elongated glass is cooled quickly to inhibit the elongated particles from returning to their original shape (respheroidizing).
The stretching technique (U.S. Pat. No. 3,653,863) is subject to several limitations. For example, the redrawing or stretching comprehends placing the glass under high tensile stress, and glass is known to be weak in tension. Consequently, although we have found that the stress necessary for good particle elongation is often at least about 3000-6000 psi, ruptures of the glass at much lower stress levels are not uncommon. A practical limit for rupture is considered to be about 1000 psi. Inasmuch as the polarizing character of the stretched glass is dependent upon the maximum stress obtained during re-drawing, premature ruptures not only interrupt the process but create undesirable rejects. Another problem not infrequently encountered when applying traditional redraw procedures to polarizing glass is related to the fact that the stretching is normally conducted at relatively high temperatures, i.e., approximating the softening point of the glass, because lower stresses can be utilized at those temperatures. This action, however, creates at least two problems with photochromic glass. The first results since the photochromic properties are quite sensitive to heat treatment. Haziness and slow fading are two commonly-experienced undesirable characteristics resulting from high temperature heating in the stretching procedure. The second is that the lower stresses are often not sufficient to elongate the particles. A further problem witnessed in the re-draw process is the difficulty in controlling the shape and size of the product. The glass to be re-drawn is customarily in the form of small bars. It is well-nigh impossible to generate wide, uniformly-thin sheets of glass, such as would be useful in ophthalmic applications, since surface flaws in the glass result in the rupturing of the draw.
Nonphotochromic glasses containing silver or other metals can be made polarizing by stretching, thereby elongating the metal particles therein, but the same problems are experienced therewith as outlined above.
U.S. Pat. No. 4,282,022 discloses a method of extruding silver-containing glasses wherein the metallic silver and/or silver halide particles within the glass are elongated and aligned in the direction of glass movement. In general, the silver and silver halide particles, prior to elongation, have diameters within the range of about 80-1000 .ANG.. The patent points out that if the extrusion process is founded essentially solely upon the presence of compressive stresses, and glass is known to be extremely strong under compression, extrusion can be carried out at very high compressive stress levels. This factor eliminates the problem of premature rupture which is encountered when applying the necessary stretching tensile stresses during a redraw operation. Thus, superior polarizing characteristics theoretically can be obtained with an extrusion process, since extrusion permits the use of higher stress levels.
The extrusion is conducted at such elevated temperatures that the glass is at a viscosity of between about 10.sup.8 -10.sup.13 poises, i.e., at temperatures between about the annealing point and the softening point of the glass, and at such pressures that the cross-sectional area of the glass in the extrusion chamber is reduced by a factor of at least 4 and up to 80 or more in the extrudate. Under the influence of those temperatures and pressures the silver and/or silver halide particles will be elongated to assume an oblate or prolate geometry with aspect ratios of at least 2:1 and up to 30:1 and greater. The photochromic glasses are also generally clear in the undarkened state and polarizing in the darkened state, while the glasses containing silver metal are always polarizing.
However, various problems are encountered with the extrusion process including a tendency for the elongated particles to return to their original shape (respheroidize) as the glass flows from the extrusion chamber, unless necessary precautions are taken. In addition, the flow of a charge in an extrusion chamber is not uniform, since the center portion thereof tends to flow more rapidly than the portion closer to the chamber and orifice walls, due to the friction of such walls. Further, the orifice serves as a heat sink for the charge which has a tendency to affect that glass closer to the walls than that centrally thereof, and accordingly the stresses generated during the extrusion process and the resulting polarizing properties are not necessarily uniform across the cross-sectional area of the extrudate. Further, although extremely high hydrostatic pressures can be developed in an extrusion chamber, it is essentially impossible to transform this pressure into an equivalent extensional stress on the particles in the glass, regardless of the extrusion orifice or die design. Also, it is extremely difficult to extrude wide, thin sheets of glass.
U.S. Pat. No. 4,304,584 describes the production of glasses exhibiting polarizing properties, i.e., glasses displaying dichroic ratios up to 40 and higher, from two types of silver-containing glasses: (1) phase separable glasses; and (2) glasses demonstrating photochromic behavior because of the presence of particles of a silver halide selected from the group of AgCl, AgBr, and AgI. The method for preparing the polarizing glasses contemplates two fundamental steps: (a) elongating or stretching the base glass articles under stress via such methods as redrawing, extruding, rolling, or stretching at temperatures between the annealing point and softening point of the glasses to cause the glass phases in the phase separable glasses or the silver halide particles in the photochromic glasses to become elongated and aligned in the direction of the stress; and (b) heat treating the elongated glass articles in a reducing environment at a temperature below the annealing point of the glasses, but above about 300.degree. C., to reduce at least a portion of the silver ions in the glass to metallic silver which is deposited in at least one of the elongated glass phases and/or along the phase boundaries of the elongated glass phases and/or deposited upon the elongated silver halide particles. The most efficient heat treatment is stated to comprise a temperature between about 375.degree.-450.degree. C. in a hydrogen atmosphere. Polarization was discerned in the visible and near infrared portions of the radiation spectrum. When photochromic glasses are made polarizing in this manner the non-reduced particles in the bulk of the glass retain their photochromic properties. Co-pending application Ser. No. 427,732, entitled Infrared Polarizing Glasses, filed simultaneously herewith, teaches how to eliminate this photochromism in cases where it is not wanted.
The U.S. Pat. No. 4,304,584 also discloses the production of composite bodies formed via concurrent extrusion of different glass compositions, such practice being operable with both phase separable glasses and photochromic glasses. In general, the composite body will consist of a thin surface layer or skin enveloping a thicker interior portion or core.
Thus, with respect to silver halide-containing photochromic glasses, it had been recognized in the art that the subjection of such glasses to high temperatures led to the growth of relatively large silver halide particles, the dimensions of the particles becoming so large as to cause light scattering with the consequent development of a hazy appearance. It was found, however, that the larger particles required less mechanical stress to effect the elongation thereof, and resisted the tendency to respheroidize to a much greater extent. Accordingly, it was deemed useful to form a laminated article comprising a thin skin glass which has been subjected to a relatively high heat treatment to generate large silver halide crystals therein and an interior portion that has been subjected to a less severe heat treatment to produce a transparent photochromic glass. Subsequent elongation of the composite body results in a thin skin exhibiting high polarization and a transparent core displaying good photochromic behavior. And, because the cross section of the skin glass is very thin, any haze developed therein will customarily have very little effect upon the optical transmission of the composite. If photochromism is not wanted, the core could of course be made of a non-photochromic glass.
Since the polarization derived from the heat treatment conducted under reducing conditions of the U.S. Pat. No. 4,304,584 is normally limited to a thin surface layer, typically 10-100 microns, the phase separable or photochromic glass need only comprise the surface layer of the composite. Hence, only the skin layer is required to be elongated uniformly, since the size, shape, and alignment of the particles in the core glass have little effect upon the polarization character of the final product.
It is apparent that the U.S. Pat. No. 4,304,584 was concerned with the utilization of relatively large silver halide particles within a thin skin glass of a composite body, since the large particles required less mechanical stress to effect the elongation thereof and further, since such particles resisted the tendency to respheroidize. However, we have observed that higher dichroic ratios of greater than 15:1 and even exceeding 60:1, representing increased polarization, are obtainable by elongating relatively small silver particles having a diameter of less than about 500 .ANG.. Further, we have learned that it is necessary to utilize particles having a diameter of less than about 500 .ANG. and preferably less than 200 .ANG. in order to eliminate the effects of light scattering, which is produced by larger particles, and which cannot be tolerated in many optical and ophthalmic applications.
We have further learned that if the particles are in the form of silver-halide, haze levels suitable for optical and ophthalmic applications can be achieved if the particle sizes are less than 200 .ANG. in diameter. Also, dichroic ratios in excess of 40 can be achieved when silver halide particles less than 600 .ANG. diameter are elongated and reduced to silver metal by heat treatment in a reducing atmosphere.
One problem, however, encountered with the utilization of such smaller particles of 500 .ANG. or less, is the fact that higher stresses, often greater than 10,000 psi, are required to elongate such particles to a degree necessary to provide the desired high dichroic ratio, which may be on the order of 40 or greater. In contrast, normal redraw operations may be performed under stresses of less than 1000 psi, and when the stress is increased above 1500 psi the glass has a tendency to rupture or break during the redraw process.
It thus has been an object of the present invention to provide an improved method of drawing or redrawing glass having silver or silver halide particles of less than about 500 .ANG. or 200 .ANG., respectively, with tensile stresses of greater than 2500 psi so as to elongate said silver or silver halide particles, hereinafter called silver-containing particles, without rupturing or breaking the glass draw.